This application is based upon and claims the benefit of priority from the prior Japanese Patent Applications No. 11-225489, filed Aug. 9, 1999; and No. 11-374989, filed Dec. 28, 1999, the entire contents of which are incorporated herein by reference.
The present invention relates to a nonaqueous electrolyte secondary battery, particularly, to a nonaqueous electrolyte secondary battery with an improved negative electrode active material.
A nonaqueous electrolyte secondary battery using a lithium metal, a lithium alloy, a lithium compound or a carbonaceous material as a negative electrode active material is expected to provide a battery having a high energy density, and vigorous researches are being made on such a nonaqueous electrolyte secondary battery.
Among the various nonaqueous electrolyte secondary batteries noted above, a lithium ion battery comprising a positive electrode containing LiCoO2 or LiMn2O4 as a positive electrode active material and a negative electrode containing a carbonaceous material capable of absorbing-desorbing lithium is widely put to practical use because the negative electrode is unlikely to be finely pulverized and it is possible to obtain a long life and a high safety.
On the other hand, a secondary battery comprising a negative electrode consisting of lithium metal, which is expected to have a high capacity, has not yet been put to practical use. It should be noted in this connection that, if lithium metal is used as the negative electrode, the negative electrode is deteriorated by the reaction between the nonaqueous electrolyte and lithium metal. Also, a lithium dendrite is generated by the repetition of the charge-discharge operations so as to cause lithium to drop from the negative electrode and, thus, to bring about an internal short circuiting. As a result, the cycle life is shortened.
In order to overcome the above-noted problems inherent in the secondary battery using a negative electrode made of lithium metal, vigorous researches are being made on the use of a lithium alloy or a lithium compound for forming the negative electrode. However, in the secondary battery comprising a negative electrode containing a lithium alloy such as lithium-aluminum alloy, it is certainly possible to suppress the reaction between the nonaqueous electrolyte and the negative electrode so as to improve the charge-discharge efficiency. However, if a deep charge-discharge is repeated, the negative electrode is finely pulverized, resulting in failure to obtain a long life. On the other hand, the discharge capacity is very small in the secondary battery comprising a negative electrode containing a lithium compound such as Li4/3Ti5/3O4.
It is also proposed to use a chalcogen compound such as an oxide or a nitride as the negative electrode active material in an attempt to further increase the capacity of the negative electrode. For example, a secondary battery comprising a negative electrode containing SnO or SnO2 as the negative electrode active material is disclosed in Japanese Patent Disclosure (Kokai) No. 7-122274 and Japanese Patent Disclosure No. 7-235293. On the other hand, a secondary battery comprising a negative electrode containing an amorphous oxide such as SnSiO3 or SnSi1-xPxO3 is disclosed in Japanese Patent Disclosure No. 7-288123. However, these secondary batteries are not sufficiently satisfactory in capacity and cycle life.
Also, a negative electrode containing an intermetallic compound such as Sn2Fe or Sn2FeC0.3 to 1.2 is disclosed on pages 405 to 427 of xe2x80x9cJ. Electrochem. Soc., Vol. 146, No. 2, 1999xe2x80x9d. It is taught that the cycle life characteristics can be improved in the presence of a mixed phase having a stoichiometric composition and consisting of Sn2Fe as a Li absorbing phase and SnFe3C as a phase that does not absorb Li. However, the discharge capacity is about 200 mAh/g, which is smaller than that of a carbonaceous material.
On the other hand, a lithium secondary battery comprising a negative electrode containing a carbonaceous material having a high crystallinity and containing at least one element M selected from the group consisting of Mg, Al, Si, Ca, Sn and Pb is disclosed in Japanese Patent Disclosure No. 9-213335 and U.S. Pat. No. 5,753,387, said carbonaceous material having a peak corresponding to the case where the interplanar spacing d002 derived from (002) reflection is 0.34 nm or less in the powder X-ray diffraction. It is taught that graphite crystallite having a sufficiently developed graphite structure is contained in the carbonaceous material. According to these documents, it is desirable for the content of the element M in the carbonaceous material to fall within a range of between 0.1 and 10 atomic %. It is described that, if the content of the element M exceeds 10 atomic %, a metal carbide is formed in an unduly large amount, resulting in failure to obtain a long life.
An object of the present invention is to provide a nonaqueous electrolyte secondary battery having a high capacity and a long life.
According to a first aspect of the present invention, there is provided a nonaqueous electrolyte secondary battery, comprising a nonaqueous electrolyte, a positive electrode, a negative electrode containing a negative electrode active material, wherein the negative electrode active material contains a composite material having a microstructure containing a phase containing carbon and a crystal phase having an average size falling within a range of between 0.01 xcexcm and 10 xcexcm.
According to a second aspect of the present invention, there is provided a nonaqueous electrolyte secondary battery, comprising a nonaqueous electrolyte, a positive electrode, a negative electrode containing a negative electrode active material, wherein the negative electrode active material contains a composite material having a microstructure containing a carbon-containing phase having an interplanar spacing d002 derived from (002) reflection of more than 0.34 nm and a crystal phase having an average size falling within a range of between 0.01 xcexcm and 10 xcexcm.
According to a third aspect of the present invention, there is provided a nonaqueous electrolyte secondary battery, comprising a nonaqueous electrolyte, a positive electrode, a negative electrode containing a negative electrode active material containing a composite material of the composition represented by formula (1);
M1xM2yC1-x-yxe2x80x83xe2x80x83(1)
where M1 is at least one element selected from the group consisting of Si, Ge, Sn, Pb, B, Al, Ga, In, Sb and Zn, M2 is at least one element having an electronegativity larger than that of Li and excluding M1 and Fe, and the atomic ratios x and y are:
0.01xe2x89xa6xxe2x89xa60.7 and 0 less than y less than 0.3.
According to a fourth aspect of the present invention, there is provided a nonaqueous electrolyte secondary battery, comprising a nonaqueous electrolyte, a positive electrode, a negative electrode containing a negative electrode active material, wherein the negative electrode active material contains a composite material having a composition represented by formula (1) and the composite material has a microstructure containing a carbon-containing phase and a crystal phase having an average size falling within a range of between 0.01 xcexcm and 10 xcexcm;
M1XM2yC1-x-yxe2x80x83xe2x80x83(1)
where M1 is at least one element selected from the group consisting of Si, Ge, Sn, Pb, B, Al, Ga, In, Sb and Zn, M2 is at least one element having an electronegativity larger than that of Li and excluding M1 and Fe, and the atomic ratios x and y are:
0.01xe2x89xa6xxe2x89xa60.7 and 0 less than y less than 0.3.
According to a fifth aspect of the present invention, there is provided a nonaqueous electrolyte secondary battery, comprising a nonaqueous electrolyte, a positive electrode, a negative electrode containing a negative electrode active material, wherein the negative electrode active material contains a composite material having a composition represented by formula (1) and the composite material has a microstructure containing a carbon-containing phase having an interplanar spacing d002 derived from (002) reflection of more than 0.34 nm and a crystal phase having an average size falling within a range of between 0.01 xcexcm and 10 xcexcm;
M1xM2yC1-x-yxe2x80x83xe2x80x83(1)
where M1 is at least one element selected from the group consisting of Si, Ge, Sn, Pb, B, Al, Ga, In, Sb and Zn, M2 is at least one element having an electronegativity larger than that of Li and excluding M1 and Fe, and the atomic ratios x and y are:
0.01xe2x89xa6xxe2x89xa60.7 and 0  less than y less than 0.3.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.